1. Field of the Invention
The present invention relates to a motor winding overheat prevention apparatus and a control apparatus for controlling drive power to be supplied to a motor.
2. Description of the Related Art
When current flows in a stator winding of a motor, loss occurs in the stator winding, and the temperature of the stator winding rises in proportion to the loss. If the temperature of the stator winding rises excessively, the stator winding will become burned. Accordingly, when driving a motor, winding overheat prevention control for preventing the burning out of the stator winding is performed by detecting the temperature of the stator winding and by stopping the driving of the motor if the detected temperature has exceeded the upper limit temperature (hereinafter referred to as “alarm level”) preset for the stator winding.
To effectively control the driving of the motor while performing control to prevent the overheating of the winding, it is indispensable to accurately detect the temperature of the winding. There are two major methods of detecting the temperature of the motor winding: one is to detect the temperature by using a temperature sensor (temperature sensing element) mounted on the stator winding, and the other is to estimate the winding temperature by calculation from the current flowing in the winding.
In one common method of estimating the winding temperature by calculation from the current flowing in the winding, the loss Q that occurs in the winding is calculated from the equation “Q=I2×R”, where I is the current flowing in the winding, which is detected by a sensing resistor, a Hall element, a current sensor, or the like, and R is the resistance of the motor winding; then, in view of the characteristic that the temperature of the stator winding rises in proportion to the loss Q, the temperature of the stator winding is estimated by multiplying the loss Q with a predetermined constant and by taking also into account the ambient temperature of the motor.
Several other methods are proposed that estimate the winding temperature more accurately by calculation from the current flowing in the winding.
For example, there is proposed a method that does not require mounting a temperature sensor on the winding but calculates the winding temperature by detecting current and voltage by using an electric circuit, as disclosed in Japanese Unexamined Patent Publication No. 2000-308389 or 2004-208453.
There is also proposed, for example, a method that measures the temperature by using a temperature sensor (temperature sensing element) mounted on the winding and that corrects the measured value by a value determined by a quadratic function of the value of the current flowing in the winding, as disclosed in Japanese Patent No. 4237075.
There is further proposed, for example, a method that detects actual winding temperature by using a temperature sensor mounted on the winding and that predicts the temperature of the winding from the detected actual winding temperature by using the load current and ambient temperature of the motor, as disclosed in Japanese Unexamined Patent Publication No. H05-268718.
The method that measures the temperature by using a temperature sensor (temperature sensing element) mounted on the stator winding has the advantage that the temperature of the stator winding can be measured accurately. However, since there is a need to efficiently conduct the heat from the stator winding to the temperature sensing element, a high-quality material having high thermal conductivity has to be used for the mounting and adhesive means for mounting the temperature sensing element on the stator winding, and there also arises a need to install wiring lines and connectors in order to transfer temperature information from the temperature sensing element to the control apparatus; hence, the disadvantage that the manufacturing cost increases.
On the other hand, the method that estimates the temperature of the stator winding by calculation from the current flowing in the winding offers the advantage of being able to reduce the manufacturing cost because of the elimination of the need for a temperature sensing element. According to this method, since the temperature of the stator winding is estimated from the loss Q (=I2×R) that occurs in the stator winding, the amount of relative change in winding temperature can be calculated accurately, but since the temperature of the stator winding is estimated by adding the amount of relative temperature change to the ambient temperature of the motor, the estimated result is affected by the ambient temperature of the motor, and therefore the thus estimated temperature of the stator winding is not sufficiently accurate.
In the winding overheat prevention control performed to prevent the burning out of the winding, since the control is performed so as to stop the driving of the motor when the detected temperature of the stator winding exceeds the upper limit temperature preset for the stator winding, the driving of the motor cannot be controlled efficiently unless the winding temperature is detected accurately.
In performing the winding overheat prevention control, when the method that detects the temperature of the stator winding by adding the ambient temperature of the motor to the amount of change in the stator winding temperature estimated based on the loss Q occurring in the stator winding is used as the method for detecting the winding temperature from the current flowing in the winding, the “alarm level”, i.e., the upper limit temperature based on which to determine whether or not to stop the driving of the motor, is usually set based on the temperature calculated by adding the ambient temperature of the motor to the amount of temperature change that the stator winding of the motor can take and on the highest ambient temperature that can be expected to occur in the operating environment of the motor; however, according to this method, it is not possible to efficiently control the driving of the motor for the following reason. FIG. 5a is a diagram showing the relationship between the temperature estimated from the current flowing in the stator winding of the motor and the ambient temperature of the motor when the ambient temperature is 40° C. FIG. 5b is a diagram showing the relationship between the temperature estimated from the current flowing in the stator winding of the motor and the ambient temperature of the motor when the ambient temperature is 20° C. FIG. 5c is a diagram showing the relationship between the temperature estimated from the current flowing in the stator winding of the motor and the ambient temperature of the motor when the ambient temperature is 0° C.
As earlier described, the temperature of the stator winding rises in proportion to the loss Q that occurs when current flows in the stator winding of the motor. Since the loss Q that occurs in the stator winding is expressed as “Q=I2×R”, where I is the current flowing in the stator winding of the motor and R is the resistance of the stator winding of the motor, a temperature change such as shown in FIGS. 5a, 5b, and 5c occurs in the stator winding. The amount of this temperature change itself is the same irrespective of the ambient temperature of the motor, but since the temperature of the stator winding is estimated by adding the amount of this relative temperature change to the ambient temperature of the motor, the estimated result varies depending on the ambient temperature of the motor. For example, the lower the ambient temperature of the motor, the lower is the temperature of the stator winding estimated from the current flowing in the stator winding. It can be assumed that the temperature calculated by adding the maximum value of the amount of temperature change occurring in the motor to the highest ambient temperature that can be expected to occur in the operating environment of the motor represents the actual upper limit temperature of the stator winding.
For example, when the highest ambient temperature that can be expected to occur in the operating environment of the motor is 40° C., it can be assumed that the temperature calculated by adding the maximum value of the amount of temperature change occurring in the motor to the ambient temperature of 40° C. represents the actual upper limit temperature of the stator winding (the temperature indicated by dashed line), as shown in FIG. 5a. Accordingly, when the highest ambient temperature that can be expected to occur in the operating environment of the motor is 40° C., the actual upper limit temperature of the stator winding is set as the alarm level for the stator winding (the temperature indicated by semi-dashed line in the figure). As a result, when the motor is driven under the highest ambient temperature of 40° C. that can be expected to occur in the operating environment of the motor, the performance of the motor can be made maximum use of without incurring the burning out of the stator winding.
However, when the motor is driven under the ambient temperature of 20° C., the alarm level for the stator winding is shifted downward from the actual upper limit temperature of the stator winding, as shown in FIG. 5b, and when the motor is driven under the ambient temperature of 0° C., the alarm level for the stator winding is shifted further downward from the actual upper limit temperature of the stator winding, as shown in FIG. 5c; as a result, the motor cannot be driven by making full use of its performance, and the efficiency thus drops.
In this way, when performing the winding overheat prevention control using the method that detects the temperature of the stator winding by adding the ambient temperature of the motor to the amount of change in the stator winding temperature estimated based on the loss Q occurring in the stator winding, with the prior art method it is not possible to efficiently control the driving of the motor.